I play percussion in a group that also has a trap-set drummer. We primarily play 4/4 rock songs. When it works for the song, we sometimes have the percussion play a rhythm in 6 (or 12) over the trap-set's rhythm in 4.  Great fun! And it really changes the feel of the song but it doesn't work for all songs. I'm not attempting to re-hash some of the older, excellent posts regarding WHAT a consonant polyrhythm is, but I would like to hear some comments regarding WHEN percussionists use the technique of 6 (or 12) vs. 4 (primarily in 4/4 rock songs).

 For example, I use a rhythm in 6 to "round out" a 4/4 song if we feel that it is beginning to sound too much like a "march". Adding the 12/8 triplets work well to give the 4/4 rhythm more of a "rolling" feel. Of course the trap-set drummer could do that as well, but the rhythms sound nice together. Another time we use polyrhythms frequently is when the song has gone into an extended instrumental, dropping back into pure 4/4 with the return of the verse: this seems to give a parallax view of the melody during the instrumental, which is a nice effect for a dreamy jam. For a more subtle effect, on some songs I play a 3/16 pattern (which is "6") while the drummer plays "4/16" pattern (of course, in 4), so while the tempo of our patterns is the same, we "meet" on the 1st down beat only after going through a few measures: this really emphasizes that 1st downbeat when we do meet and that is a cool effect (we call that a "cross-over" pattern). Yet for other songs, the bass player has some 4/4 grooves that travel well with a 6/8 bell pattern throughout the entire 4/4 song. Usually we just experiment with the polyrhythm to see if it works, and when it does we can't really articulate why we like it or why it works. So I thought that I would ask you guys.

I understand that music is an art, not a science, and to some extent "if it sounds good, play it", but is there any theory, strategy or guidelines as to when interjecting a rhythm in 6 works particularly well for a 4/4 song? What are the overall effects achieved by doing so? In what situations does this work for you?  When do you polyrhythm?

This doesn't always work and has nothing to do with song choice. Why? Because Hip Hop and the way the drummer is playing the swung sixteenth-notes isn't always based on three note groupings.

Just like any shuffle ... it CAN be pure triplet based, but it can also be straight sixteenth based (dotted-eighth, sixteenth), as well as somewhere in between. If you go trying to play a 12/8 feel with this ... the only way it will feel right is if the drummer is playing his Hip Hop / Shuffle feel as pure, exact, triplet based, shuffle.

If the beat is sub-divided, as opposed to just divided, more notes are being played, which means if you start playing a triplet based groove over a straight duple groove, it will get  muddy really fast. Instead of 2:3 it's now 4:3 ... and at dance tempos ... it will be a train wreck.

Ultimately, if it sounds and feels good ... do it ... don't worry about the math or whether it makes since or not.